function phi = phi4(spectrum,m,n,x,y)
LatticeSize = spectrum.LatticeSize;
XYmax = spectrum.XYmax;
if(m==0||2*m==LatticeSize)
    coeff = sqrt(2)/XYmax;
else
    coeff = 2.0/XYmax;
end
phi = coeff*cos(2*m*pi*x/LatticeSize)*sin(2*n*pi*y/LatticeSize);